Hitherto, a heat-sensitive recording material of obtaining colored images by contacting a color former and a color developer by the action of heat by utilizing the reaction of both components. Since such a heat-sensitive recording material is relatively inexpensive and also the recording device is compact and requires faily easy maintenance, the recording material is used not only as recording media for facsimile and various kinds of computers but also for other wide field of arts such as heat-sensitive labels, etc.
However, a heat-sensitive recording material has weak points in fingerprint resistance and solvent resistance. For example, when the recording layer thereof is brought into contact with a sebum of finger or a solvent, the recording density is reduced or an unnecessary color called as background fog forms.
For solving such a problem, there are proposed a method of coating an aqueous emulsion of a resin having a film-forming faculty and a chemical resistance on the heat-sensitive recording layer as disclosed in JP-A-54-128347 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a method of coating a solution of a water-soluble polymer such as polyvinyl alcohol, etc., on the heat-sensitive recording layer as disclosed in JP-A-U-56-125354 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application"), etc. However, the improvement by these methods is accompanied by new disadvantages and satisfactory results are not always obtained at present.
For example, when a coating of the aqueous resin is applied on the heat-sensitive recording layer, it is required to restrict the drying temperature for preventing coloring of the recording layer with a high temperature drying, whereby curing of the resin layer is, as a matter of course, insufficient to cause a phenomenon of sticking the resin layer to a recording head druing the recording.
Accordingly, it has been proposed to coat a resin component capable of being cured by electron beams on the heat-sensitive recording layer and cure the resin component by electron beams. However, such a method is yet insufficient in the preservability of recorded images. In addition, there may also be problems that the electron beam-curable resin layer causes coloration of the heat-sensitive recording layer just after it has been coated or causes fading of recorded images.
As the result of various investigations for solving these problems, the inventors previously discovered that by forming an interlayer of an aqueous resin on the heat-sensitive recording layer and thereafter forming thereon an overcoat layer containing a resin that is curable upon exposure to electron beams, a heat-sensitive recording material showing improved preservability of recorded images without being accompanied by the formation of fog on the recording layer, having widely varying surface characteristics, and having excellent recording characteristics was obtained as shown in JP-A-62-279980.
On the other hand, recently, various printers such as video printers, etc., which can provide high quality images comparable to photographs are used and even with regard to a heat-sensitive recording material for print out, more excellent recording density and gradation have been required. For the purpose, the development of heat-sensitive recording materials using a plastic film or a synthetic paper as the support and having excellent gradation of recorded images has been made and for improving the preservability of recorded images on such a heat-sensitive recording material, it has been attempted to form an overcoat layer of an aqueous resin, etc., on the recording layer. However, when the heat-sensitive recording material having the overcoat layer composed of an aqueous resin is used for recording by a video printer under a high humidity condition, it has been found that the overcoat layer tends to stick to a recording head or paper feed guides to cause paper clogging. Also, the record density is still insufficient.
As the result of various investigations on such problems, the inventors further discovered that by forming a heat-sensitive recording layer on a plastic film or a synthetic paper, forming an interlayer of an aqueous resin on the heat-sensitive recording layer, and then forming an overcoat layer containing a resin curable upon exposure to electron beams followed by curing by electron beams, a heat-sensitive recording material giving high recording density, having excellent gradation and preservability of recorded images, and causing no sticking onto a recording head even under a high humidity condition was obtained and previously filed as European Patent No. 264,827A.
However, it has been clarified that when even the recording material having excellent characteristics as described above is used for recording by a video printer of printing at a particularly high speed, a sticking phenomenon to a recording head still occurs even by using the electron beam-curable resin as the resin for forming the overcoat layer to cause a problem in the paper feed property or sticking (a heat-sensitive recording material sticks to a thermal head and is not fed smoothly). Such a problem can be improved to some extent by the addition of a pigment to the overcoat layer but the aforesaid improvement is still insufficient in paper feed property in a particularly highhumidity condition or a condition of attaching moisture or a sebum of human skin onto the surface of the recording material.